The present invention relates to a process for the electrolytic transformation of furan or one or more furan derivatives.
An objective of preparative organic electrochemistry is to utilize the processes occurring at the two electrodes in an electrochemical process in parallel. Processes in which the two electrode processes which occur in an undivided cell can be utilized for the reaction of chemical compounds are of particular interest.
An example of such a process is the oxidative dimerization of 2,6-dimethylphenol, which is coupled with the dimerization of maleic esters (M. M. Baizer, in: H. Lund, M. M. Baizer (Eds), Organic Electrochemistry, Marcel Dekker, New York, 1991, pages 1442 ff.).
A further example is the coupled synthesis of phthalide and t-butylbenzaldehyde (DE 196 18 854).
However, it is also possible to utilize the cathode process and the anode process to produce a single product or to decompose a starting material. Examples of such electrochemical processes are the production of butyric acid (Y. Chen, T. Chou, J. Chin. Inst. Chem. Eng. 27 (1996) pages 337-345), the anodic dissolution of iron, which is coupled with the cathodic formation of ferrocene (T. Iwasaki et al., J. Org. Chem. 47 (1982) pages 3799 ff.) or the decomposition of phenol (A. P. Tomilov et al., Elektrokhimiya 10 (1982) page 239).
An example of a process in which a furan derivative is reacted in an undivided electrolysis cell and the two electrode processes are utilized is the oxidation of furancarboxylic acid with subsequent ring opening to form 1-carboxymethyl-4,4-dimethoxypropene which is hydrogenated in a further step to give the saturated propane derivative (T. Iwasaki et al., J. Org. Chem. 47 (1982) pages 3799 ff.). However, this is not a catalytic hydrogenation, but a direct electroreduction. In this case, however, it is not the furan which is reacted but the xcex1,xcex2-unsaturated ester, i.e. a class of substances whose electrochemical reduction is known. Furthermore, the ring opening and the subsequent hydrogenation do not occur directly on the anodically generated product but on a fragment which has undergone further oxidation and has one less carbon atom.
An electrochemical oxidation of furan or a furan derivative with retention of the heterocyclic ring structure and subsequent hydrogenation involving hydrogenation of a double bond which is present in the ring structure after oxidation is, however, not known in processes in which both electrode processes are utilized.
It is an object of the present invention to provide an electrochemical process which preferably proceeds in an undivided electrolysis cell and in which furan or a substituted furan is oxidized in one electrode process with retention of the heterocyclic ring structure and this oxidation product is hydrogenated by means of hydrogen which is formed as product in the other electrode process or is transferred as hydrogen equivalent to the furan derivative in the sense of an electrocatalysis.
We have found that this object is achieved by a process for the electrolytic transformation of at least one furan derivative (A) in an electrolysis circuit which comprises both the steps (i) and (ii):
(i) electrolytic oxidation of furan or a substituted furan or a mixture of two or more thereof to give
(a) at least one furan derivative (B) which has a Cxe2x80x94C double bond in the five-membered heterocyclic ring, and
(b) hydrogen;
(ii) hydrogenation of this Cxe2x80x94C double bond using the hydrogen obtained in parallel at the cathode in step (i) or hydrogen fed to the electrolysis circuit from outside or electrocatalytic hydrogenation,
wherein the process is carried out in an electrolysis cell in which at least one hydrogenation catalyst is present.
The process preferably occurs in an undivided electrolysis cell.
Apart from furan, the following compounds may be mentioned as examples of preferred substituted furans: fural(furan-2-aldehyde), alkyl-substituted furans, furans substituted by xe2x80x94CHO, xe2x80x94COOH, xe2x80x94COOR, where R is an alkyl, benzyl or aryl group, in particular a C1-C4-alkyl group, xe2x80x94CH(OR1)(OR2), where R1 and R2 may be identical or different and are each an alkyl, benzyl or aryl group, in particular a C1∴C4-alkyl group, and xe2x80x94CN groups in the 2, 3, 4 or 5 position.
In the reaction according to the present invention of organic compounds, it is possible to use solvents and electrolyte salts as are described in H. Lund, M. M. Baizer, (Eds) xe2x80x9cOrganic Electrochemistryxe2x80x9d, 3rd Edition, Marcel Dekker, New York 1991.
The oxidation is, according to the present invention, preferably carried out in the presence of methanol or in the presence of ethanol or a mixture thereof, but preferably in the presence of methanol. The substrates can simultaneously act as reactant and solvent.
Solvents which can be used in the reaction include not only furan and substituted furan and the compound used for the oxidation but also all suitable alcohols in general.
Electrolyte salts which can be used in the process of the present invention include NaBr and also, for example, alkali metal and/or alkaline earth metal halides, with bromides, chlorides and iodides being possible as halides. Ammonium halides can likewise be used.
Pressures and temperatures can assume values as are customarily employed in catalytic hydrogenations.
In a preferred embodiment of the process of the present invention, the reaction temperature T is  less than 50xc2x0 C., preferably  less than 25xc2x0 C., the pressure p is  less than 3 bar and the pH is in the neutral region.
In a preferred embodiment of the process of the present invention, intermediates are fed in in addition to the starting materials which are introduced into the undivided electrolysis cell. The term intermediate refers to a product which is obtained as furan derivative (B) in step (i) of the above-described process by electrolytic oxidation of furan or a substituted furan or a mixture of two or more thereof and is therefore present in the electrolysis circuit. The concentration of the additional intermediates is set by means of customary electrochemical and electro-catalytic parameters, for example current density, type of catalyst and amount of catalyst or the intermediate is added to the circuit
As regards the specific choice of the electrode material, there is no restriction in the process of the present invention as long as the electrodes are suitable for the above-described process.
Preference is given to using graphite anodes in the undivided cell.
As far as the geometry of the electrodes in the undivided electrolysis cell is concerned, there are essentially no restrictions in the process of the present invention. Preferred geometries are, for example, flat parallel electrode arrangements and annular or cylindrical electrode arrangements.
In a preferred embodiment of the invention, at least one electrode is in contact with at least one hydrogenation catalyst. In a particularly preferred embodiment, the at least one hydrogenation catalyst is part of a gas diffusion electrode. In a further preferred embodiment of the invention, at least one electrode is a graphite electrode which is coated with noble metal and is in the form of a plate, mesh or felt. In another preferred embodiment of the invention, the hydrogenation catalyst in the form of a suspension in the electrolyte is continually brought into contact with at least one electrode. Here, the hydrogenation catalyst, i.e. the catalytically active material, is pumped around in the cell or washed onto an appropriately structured cathode or anode. Such a wash-coated electrode is described, for example, in DE 196 20 861.
If a gas diffusion electrode is used for at least one of the electrodes, the material from which the gas diffusion electrode is made can in principle have been processed so that the gas diffusion electrode can be used as electrode without support material. In a preferred embodiment, an alternative is provided by at least one of the electrodes used being a composite which comprises at least one conventional electrode material and at least one material for a gas diffusion electrode.
It is conceivable for this further electrode material to comprise one electric conductor or a plurality of electric conductors.
It is in principle conceivable for the composite comprising the conventional electrode material and the material of the gas diffusion electrode to be used as one electrode in the process of the present invention together with one or more suitable counterelectrodes.
These one or more suitable counterelectrodes are subject to no restrictions in respect of their geometry and their chemical composition, as long as the process of the present invention can be carried out using them.
In a preferred embodiment of the process of the present invention, the further electrode material which forms a composite with the gas diffusion electrode material is also used as counterelectrode to the gas diffusion electrode. This is achieved by a bipolar electrode arrangement.
In a preferred embodiment of the process of the present invention, graphite and/or carbon fiber paper are/is used as base material for the gas diffusion electrode. The catalyst composition is applied thereto.
As support material on which the gas diffusion electrode material is supported, preference is given, in the process of the present invention, to further electrode materials comprising carbon.
In the process of the present invention, a Cxe2x80x94C double bond is, as described above, hydrogenated electrocatalytically using the hydrogen obtained in step (i) or using the corresponding hydrogen equivalents in the sense of an electrolysis. In this hydrogenation, the compound to be hydrogenated is preferably brought into contact with one or more hydrogenation catalysts.
As regards the choice of hydrogenation-active catalysts, there are in principle no restrictions for the purposes of the process of the present invention. All catalysts known from the prior art can be used. Mention may be made, inter alia, of the metals of transition groups I, II and VIII of the Periodic Table, in particular Co, Ni, Fe, Ru, Rh, Re, Pd, Pt, Os, Ir, Ag, Cu, Zn and Cd.
For example, it is possible to use the metals in finely divided form. Examples are Raney Ni, Raney Co, Raney Ag and Raney Fe, each of which may further comprise other elements such as Mo, Cr, Au, Mn, Hg, Sn or S, Se, Te, Ge, Ga, P, Pb, As, Bi or Sb.
It is naturally also possible for the hydrogenation-active materials described to comprise a mixture of two or more of the hydrogenation metals mentioned, which may, if desired, be combined with, for example, one or more of the abovementioned elements.
Of course, it is also conceivable for the hydrogenation-active material to be applied to an inert support. As such support systems, it is possible to use, for example, activated carbon, graphite, carbon black, silicon carbide, aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide or mixtures of two or more thereof, e.g. as a suspension or as finely divided granulated material.
In a preferred embodiment of the present invention, the hydrogenation-active material is applied to the base material of the gas diffusion electrode.
Accordingly, the present invention also provides a process as described above in which the base material of the gas diffusion electrode is laden with a hydrogenation-active material.
As hydrogenation-active material with which the gas diffusion electrode system is laden, it is possible to use all hydrogenation catalysts as described above. Of course, it is also possible to use a mixture of two or more of these hydrogenation catalysts as hydrogenation-active material.
It is naturally also conceivable in the process of the present invention for the gas diffusion electrode material to be laden with hydrogenation-active material and for use to be made of additional hydrogenation-active material which is identical to or different from that with which the gas diffusion electrode material is laden.
The process of the present invention, as described above, in principle allows, in particular, a choice between using the electrocatalytically active electrode, i.e. the electrode which is in contact with a hydrogenation catalyst, as cathode or as anode or as cathode and anode.
The present invention therefore also provides a process as described above in which the electrocatalytically active electrode, for example a gas diffusion electrode, is used as cathode and/or as anode.
Furthermore, the present invention provides a process as described above in which the furan derivative (B) produced is reacted to form at least one ring-opened butane derivative. The ring-opened butane derivative is preferably 1,1,4,4-tetrarnethoxybutane or a substituted 1,1,4,4-tetramethoxybutane.